Businesses, governmental organizations and other entities are increasingly saving large volumes of data necessary for daily operations. In order to provide proper protection of data for business and legal purposes (e.g., to ensure quick recovery of data in the event of a disaster or to comply with document retention requirements), entities often back up data to a physical media, such as magnetic tapes, on a regular basis. Traditional backup systems placed an application server, backup server, source device, destination device and a local area network (“LAN”) in the data path of backup operations. Under these systems, the LANs were becoming overburdened by the amount of data being copied and often the backup window was too short to achieve a complete backup of data. Many entities have now implemented Storage Area Networks (“SAN”) to relieve much of the burden of mass data storage and backup from the LAN, freeing the LAN for more immediate data storage and manipulation operations.
Many SANs utilize a Fibre Channel-to-SCSI architecture in which SCSI storage devices are connected to Fibre Channel-to-SCSI routers. This allows data to be transferred across the SAN with the speed of Fibre Channel while using well defined SCSI commands. The T10/99-143r1, “Working Draft SCSI Extended Copy Command” (the “99-143r1 Draft”) and NCITS T10 SPC-2 (SCSI Primary Commands-2) (“SPC-2”) provide a mechanism for computer backup application to delegate actual data movement to third party devices known as “copy manager devices” or “data mover devices”.
The copy manager devices move data from source devices to destination devices as designated by the backup application in “segment descriptors” which in part constitute the parameter list of an extended copy command. The SPC-2 standard contemplates that a copy manager device may process some number of concurrent extended copy commands that specify different destination devices.
The extended copy command is often used to backup data from random access devices, such as hard drives, to sequential access devices, such as tape drives. When the backup medium is a tape, the copy manager device strives to keep the tape moving by streaming data to the tape in order to maximize performance. To keep the drive streaming, copy manager devices often use some form of disk data pre-fetch, or “read ahead,” so that the copy manager has enough data in its memory to build the next tape write command when an active tape write command completes.
With the advent of new, higher-speed, tape drives, copy manager devices have difficulty pre-fetching enough data from source devices to effectively keep the tape drive streaming. This reduces the efficiency of backup operations and can create data gaps on a tape.